Sol-gel/hydrothermal two-step synthesis strategy for promoting Ag species–modified TiO₂-based composite activity toward H₂ evolution under solar light

Highlights

• A one-pot sol-gel/hydrothermal two-step (SH II) synthesis strategy was developed.

• The optimized SH II composite showed better physicochemical properties and activity.

• The SH II method can control metallic Ag formation and realize the negative shift of the conduction band.

• Methanol possessing efficient hole capture ability greatly enhanced H₂ evolution.

• SH II synthesis is a promising strategy to further improve the composite's photocatalytic ability.

Abstract

In this work, a facile one-pot sol-gel/hydrothermal two-step (SH II) process was developed to synthesize the P/Ag/Ag₂O/Ag₃PO₄/TiO₂ composite photocatalyst with superior H₂ evolution ability under solar light. Compared with the one-step sol-gel method, a negative shift of the conduction band edge (−0.51 eV vs. NHE) over H⁺/H₂ evolution potential was successfully achieved in SH II process by controlling the amount of metallic Ag formation under optimal conditions, which subsequently enabled the SH II–generated electrons to react with H⁺ to produce H₂. Moreover, the smaller crystal size, bigger specific surface area, faster charge carrier movability, and lower recombination rate in the SH II material played a critical role in increasing H₂ evolution. In addition, the H₂ generation rate of the SH II composite was further promoted by coupling with photocatalytic oxidation of alcohols, more specifically with methanol to efficiently consume photogenerated holes. Furthermore, higher H₂ yield was sustainably maintained even after ten recycling runs. Therefore, based on the advantages of simple operation and high efficiency, the SH II method could be a promising strategy to further improve the ability of composite photocatalysts for practical application in wastewater treatment and the water splitting field in the future.

Introduction

At present, more than 80% of the world's energy is supplied by fossil fuels, which have limited reserves [1]. The need to solve the global energy crisis and environmental concerns has led researchers to work on development of the hydrogen (H₂) economy [2]. H₂ is regarded as a green energy source and a promising environmentally friendly alternative to fossil fuels. Among various technologies for H₂ generation, photocatalytic water-splitting process is considered one of the most efficient methods for breaking water into H₂ [3,4]. Since Fujishima and Honda [5] discovered TiO₂ photocatalysis, it has attracted worldwide attention owing to its relatively high activity, low cost, non-toxicity, and chemical-physical stability. In the past decades, TiO₂ photocatalyst has been applied for water purification, disinfection, and H₂ generation [6,7]. However, practical application of TiO₂ is limited owing to large bandgap energy [8], high recombination rate of photogenerated electron-hole pairs [9], and low H₂ production in pure water [10]. To overcome these drawbacks, several methods have been developed to enlarge the light response region and improve the activity of TiO₂ photocatalyst, including modification by dopants and using different systemization processes.

Doping of metal and metal salts (Ag, Pt, and Au [[11], [12], [13]]) has been applied in several studies to improve photocatalytic performance in practical application. In our previous studies [[14], [15], [16]], it has been shown that with suitable species of metal and non-metal dopants (P, Ag, Ag₂O, and Ag₃PO₄), photocatalytic activity of TiO₂ photocatalyst was highly promoted because the low recombination rate of electron-hole pairs and higher light absorbance in the visible range was caused by addition of dopants.

In another aspect, it has been demonstrated that physicochemical properties and activity of photocatalysts are mainly determined by the preparation methods and conditions [17,18]. Photocatalytic semiconductors can be synthesized by different synthetization methods, such as sol-gel process, hydrothermal method, direct oxidation reactions, chemical precipitation method, light deposition method, and so on [18]. Among these methods, the sol-gel method has often been used in preparation of semiconductor photocatalysts. Owing to its ability of homogeneous mixing of dopants at the molecular level, it is a useful method for modifying TiO₂ using metal or non-metal dopants [18]. However, the calcination process in the sol-gel method is necessary for the formation of the crystalline material, which frequently leads to serious grain growth, particle agglomeration, and small surface area [19]. On the other hand, the hydrothermal method has gained increasing attention owing to its ability to crystallize titanium dioxide at mild temperatures (100–300 °C). The hydrothermal method is normally conducted in autoclaves with Teflon liners under controlled temperature and pressure, with the reaction in aqueous solutions. Owing to high pressure in the sealed reactor, the temperature of the solution can be elevated higher than the boiling point, reaching the pressure of vapor saturation. Under these circumstances, photocatalysts with small crystal size and high specific surface area can be synthesized by optimizing hydrothermal conditions (temperature and time). As such, coupling sol-gel process with hydrothermal treatment instead of calcination could be an attractive synthesis strategy to obtain composites with higher photocatalytic activity. Velázquez-Martínez et al. [20] demonstrated that iron-doped TiO₂ powder with high specific area can be obtained by the sol-gel method combined with hydrothermal treatment. Similarly, Fhoula et al [21] have reported an increase in activity of Eu³⁺:TiO₂ composite prepared by hydrothermal-assisted sol-gel processes. On the other hand, Ag species–modified TiO₂ composites are generally synthesized by the sol-gel or hydrothermal method, respectively [22,23]. Until now, studies on their preparation via sol-gel/hydrothermal combined methods are still lacking.

Besides material modification and fabrication technology to achieve higher efficiency of photocatalytic water splitting, many studies have shown the potential use of electron donors (hole acceptors) as sacrificial agents to facilitate water reduction, which can react irreversibly with the formed photogenerated holes [24]. But if the sacrificial donors are more expensive than the produced H₂, the use of electron donors is not of great value. A good strategy is to use organic wastes or pollutants in water as electron donors. This will result in production of a clean energy source, H₂, with simultaneous removal of environmental pollutants. As such, to further enhance photocatalytic H₂ production performance, there have been some studies published in the literature on the use of alkanes and alkenes, glucose, phenol, organic acids, and dyes as sacrificial agents, but the results were not as efficient as those of the reports using alcohols [[25], [26], [27]]. Considering, in current industry, alcohols (methanol and ethanol) as being widely used, basic organic chemicals are abundantly found in waste material (sewage from wine, paper, or daily chemical industry), which have caused severe environmental pollution. Therefore, using methanol or ethanol as sacrificial agents could be a promising cost-efficient strategy to promote photocatalytic H₂ evolution, especially in future for large-scale sustainable photocatalytic H₂ production assisted by sunlight.

In the previous study of our laboratory, the sol-gel–synthesized P/Ag/Ag₂O/Ag₃PO₄/TiO₂ (PAgT) photocatalyst has presented high activity for organic pollutant degradation and bacteria sterilization under solar light [[14], [15], [16]]. However, the specific surface area of the sol-gel–synthesized photocatalyst is relatively small, and the conduction band (CB) edge is more positive than H⁺/H₂ evolution potential, which is not beneficial for water splitting. In addition, the high temperature (400 °C) and long-term (2 h) calcination treatment in the sol-gel synthesis method may decompose the light-sensitive structure of Ag salts in the PAgT composite. As previously mentioned, it is well known that the preparation method largely determines the physicochemical properties and activity of the photocatalyst. From this point of view, it seems possible to further improve the activity of the PAgT composite photocatalyst by changing the synthesis procedure with the same Ag species as dopants. Clarifying this point will have far-reaching significance for further promoting the current composite photocatalytic activity.

Therefore, in this study, the one-step sol-gel (S I)–synthesized PAgT photocatalyst was further improved by the hydrothermal method. To determine the appropriate preparation conditions of the one-pot sol-gel/hydrothermal two-step (SH II) synthesis strategy, hydrothermal temperature and time were optimized. Then, the structure, morphology, and chemical properties of the prepared photocatalysts obtained by two different routes (S I and SH II) were characterized. In addition, the influence of alcohols on photoactivity of SH II in H₂ production by water splitting was also presented. Finally, the possible mechanism of the enhanced SH II photocatalytic activity was systematically investigated and proposed.